The present invention relates to a method for increasing the control dynamics of a load driven by a driveshaft of a direct drive, and more particularly to a method that suppresses resonances.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
In conventional control applications, the rotation speed of a direct drive, which is used as an input variable for controlling the driven load, is typically controlled by measuring the rotation speed either close to the load or close to the direct drive. In a direct-drive system, the driveshaft is directly mechanically connected both with the rotor of the direct drive and also with the driven load. Accordingly, no gear which could have slack and/or suppleness is interposed between the direct drive and the load.
Because the driveshaft has a finite stiffness, the driveshaft can vibrate during operation, for example, perform torsional oscillations. As a result, the rotation speed along the driveshaft is not constant. Depending on the location where the sensor measuring the rotation speed is located, the value of the measured rotation speed can vary more or less due to oscillations of the driveshaft. The amplitude of the oscillations can be particularly high in frequency ranges where mechanical resonances occur. The achievable control dynamics is particularly impaired in direct drive systems, where the dynamics of the system is determined by an adjustable amplification of the controller. The oscillations in the measured rotation speed of the driveshaft caused by the driveshaft oscillations must be filtered out in order to prevent control instabilities, before the rotation speed can be supplied as an actual control variable to a controller that controls the rotation speed of the driveshaft. Filtering which is typically performed in the range of the mechanical resonance frequency, can reduce the control dynamics. However, the control circuit will only be stable when the amplification of the control circuit is reduced.
To achieve an optimal control, a transducer should be arranged in the region of a so-called oscillation node on the driveshaft, i.e., in a region where the amplitude of oscillations has a minimum. This would obviate the need for filtering.
In practical applications, however, it is frequently not possible to attach a transducer near the oscillation node due to physical constraints. Moreover, the position of the oscillation node on the driveshaft can shift with the size of the load.
Methods for increasing the control dynamics of a drive train of a machine tool or production machine with backlash and/or suppleness are known. For example, the drive train can include a gear with backlash and/or suppleness, and the motor rotation speed and the load rotation speed are measured on both sides of the gear.
It would therefore be desirable and advantageous to provide an improved method for increasing the control dynamics of a load driven by a driveshaft of a direct drive, which obviates prior art shortcomings and is able to specifically operate with a high amplification without the need for a notch filter to filter out resonances.